<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .outer {
            border: 5px solid black;
        }
        
        .box {
            padding: 10px;
            width: calc(90% - 30px);
            background-color: rebeccapurple;
            color: white;
        }
    </style>
</head>

<body>
    <div class="outer">
        <div class="box">The inner box is 90% - 30px.</div>
    </div>
</body>

</html>